Near-end crosstalk noise minimization and power reduction for digital subscriber loops

ABSTRACT

To optimize the performance of DSL modems in the same cable bundle, the size and position of the group of subcarriers used for transmission is intelligently selected when the bit rate necessary for making the transmission is less than the total available bandwidth provided by all subcarriers. By intelligently selecting a minimum number of subcarriers for Digital Multi-tone (DMT) signal transmission, a reduction in line driver power consumption is effectuated. Additionally, by intelligently selecting the position of the groups of subcarriers within the total available subcarriers, near-end crosstalk (NEXT) noise within the cable bundle may be minimized.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.10/029,190, filed Dec. 19, 2001, now U.S. Pat. No. 7,126,984, issuedOct. 24, 2006, the disclosure of which is hereby incorporated byreference.

The present application is related to U.S. application Ser. No.10/028,805 filed Dec. 19, 2001 and entitled “METHOD AND APPARATUS FORAPPLICATION DRIVEN ADAPTIVE DUPLEXING OF DIGITAL SUBSCRIBER LOOPS”, thedisclosure of which is hereby incorporated by reference

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to digital subscriber loop (DSL) systemsand, more particularly, to a method and apparatus for minimizingnear-end crosstalk (NEXT) noise and reducing power consumption within aDSL system modem, transmitter and line driver.

2. Description of Related Art

Crosstalk is noise that is present on a phone line due to theelectromagnetic radiation of other, closely proximate, phone lines (forexample, lines located in the same cable bundle). The term “crosstalk”was originally coined to indicate the presence in a telephone receiverof unwanted speech sounds from another telephone conversation. The termhas been gradually broadened in application to refer to interferencebetween any kind of communications circuits. This kind of noise includesboth near-end crosstalk (NEXT) and far-end crosstalk (FEXT) components.

With respect to digital subscriber loop (DSL) systems, it is generallyaccepted that the NEXT noise, as opposed to FEXT noise, presents themajor source of interference. The reason for this is that FEXT noisepasses through the entire DSL loop and thus its propagation lossgenerally is very large and in many cases the noise can simply beignored. The opposite is true with respect to NEXT noise which undergoeslittle, relatively speaking, attenuation in its short propagation path.The concerns over NEXT noise remain even when the bit rate of thetransmitted signal is small because idle ATM cells are inserted to fillup all the data frames of the DSL link (both upstream and downstream),and the transmission of this filler material is also a source of noise.

When DSL services are offered on different loops in the same cablebundle, it is very important to reduce and minimize NEXT noisecontributed by a DSL communication on one loop with respect to thecommunications on other loops within the bundle. Doing so beneficiallyimproves DSL system error rate performance and increases loopthroughput.

Power consumption is also a very important factor to be managed in DSLsystems. This is most commonly an issue raised with respect to thedesign of the DSL modem, and it applies to both the customer premisesequipment (CPE) location and the central office (CO) location. A numberof power concerns are recognized in the art. For example, the more powerthat is transmitted in a DSL system, the more likely it is thatcrosstalk noise will be coupled to other DSL users in the same cablebundle. It is also recognized that if a universal serial bus (USB)interface is used for an external modem at the CPE side, the powerconsumption of the modem is limited by the USB standard. With respect tothe CO location, many DSL line cards are installed in a very limitedspace, and heat dissipation is a serious concern. Any reduction in powerconsumption in the DSL modem is therefore welcomed. Still further, powerconsumption is also important for laptop computers having limitedcapacity batteries. Finally, the use of additional bandwidth by thefiller material ATM idle cells (which may lead to NEXT noise asdiscussed above) increases the power consumption for both of the linedrivers at the CO and CPE locations without providing a substantivecommunications benefit.

SUMMARY OF THE INVENTION

In an embodiment, a method and system for optimizing digital subscriberline (DSL) communications performance over a cable bundle having aplurality of loops and including at least one active DSL loop,comprises: determining a required bit rate of a DSL loop communication,the determined required bit rate corresponding to a group of pluralsubcarriers numbering less than a total available subcarriers on one ofthe plurality of loops; calculating, for a plurality of subcarrierlocation positions of the group of plural subcarriers for the DSL loopcommunication within the total available subcarriers, a crosstalk noiseeffect of the DSL loop communication with respect to the at least oneactive DSL loop; and choosing a location position for the group ofplural subcarriers to carry the DSL loop communication within the totalavailable subcarriers where the calculated crosstalk noise effect withrespect to the at least one active DSL loop is minimized.

The subcarriers may vary in number with different potential positions ofthe required bandwidth within the total available bandwidth.

The process and system are applicable to both upstream and downstreamavailable bandwidths.

The group of subcarriers may comprise a group including at least twoadjacent subcarriers.

The group of subcarriers may comprise a group including at least twonon-continuous subcarriers

In another embodiment, a digital subscriber line (DSL) transmitter isconnected to a certain loop in a cable bundle having a plurality ofother loops and including active DSL loop communications on the otherloops. A DSL loop communication on the certain loop needs a group of DMTsubcarriers less than a total available number of DMT subcarriers onthat certain loop. The transmitter comprises: a noise estimationalgorithm that is operable to calculate, at each one of a plurality ofpossible subcarrier positions of the group of DMT subcarriers within thetotal available number of DMT subcarriers, a crosstalk noise effect ofthe DSL loop communication with respect to the active DSL loopcommunications on the other loops; a noise minimization algorithm thatis operable to choose one of the possible subcarrier positions as alocation of the group of DMT subcarriers within the total availablenumber of DMT subcarriers, wherein the calculated crosstalk noise effectwith respect to the active DSL loop communications on the other loops atthe chosen one of the possible positions is minimized; and a DSL signalgenerator for generating the DSL loop communication using the group ofDMT subcarriers which are positioned at the chosen location.

In accordance with another embodiment, a method for optimizing digitalsubscriber line (DSL) communications performance over a cable bundlehaving a plurality of loops and including at least one active DSL loop,comprises a) determining a required bit rate of a DSL loopcommunication, b) identifying a group of plural subcarriers starting ata subcarrier location position and numbering less than a total availablesubcarriers on one of the plurality of loops which have a capacity forhandling the required bit rate, and c) calculating at the subcarrierlocation position of the identified group of plural subcarriers for theDSL loop communication a crosstalk noise effect of the DSL loopcommunication with respect to the at least one active DSL loop. Steps b)and c) are repeated to obtain crosstalk noise effect at a plurality ofsubcarrier location positions for the DSL loop communication. One of thesubcarrier location positions is then chosen where the calculatedcrosstalk noise effect with respect to the at least one active DSL loopis minimized as the starting location within the total availablesubcarriers of the corresponding identified group of plural subcarriersto carry the DSL loop communication.

By choosing a possible position for the group of subcarriers within thetotal number of subcarriers where the calculated crosstalk noise effectwith respect to the at least one active DSL loop is minimized, anoptimized performance for the DSL modem may be achieved. Furthermore,the use of a minimum number of DMT subcarriers in association with thegroup effectuates a reduction in line driver power consumption ascompared to the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and apparatus of the presentinvention may be acquired by reference to the following DetailedDescription when taken in conjunction with the accompanying Drawingswherein:

FIG. 1 is a functional block diagram of an ATU-R transmitter inaccordance with an embodiment of the present invention;

FIG. 2 is a functional block diagram of an ATU-C transmitter inaccordance with an embodiment of the present invention;

FIG. 3 is a diagram illustrating NEXT noise and FEXT noise sources in acable bundle;

FIG. 4 is flow diagram illustrating a process for NEXT noiseminimization when establishing a new DSL link;

FIG. 5 is a state machine diagram operating an idle ATM cell removalprocess;

FIG. 6 is flow diagram illustrating a process for performing an idlecell discarding operation;

FIG. 7 illustrates selective bandwidth utilization for DSL service;

FIG. 8 illustrates selective bandwidth utilization to minimize NEXTnoise in a non-overlapped DSL system implementation;

FIG. 9 illustrates selective bandwidth utilization to minimize NEXTnoise in an overlapped DSL system implementation; and

FIG. 10 is a flow diagram for a process to minimize NEXT noise for thenew initialized loop in the same cable bundle.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is now made to FIG. 3 wherein there is shown a diagramillustrating near-end crosstalk (NEXT) noise and far-end crosstalk(FEXT) noise sources in a cable bundle 10. It is well known that theperformance of a DSL modem is generally limited by the crosstalk noiseintroduced by other modems that are connected to the other loops in thesame cable bundle 10. The crosstalk phenomenon can be modeled using twocomponents, namely NEXT noise and the FEXT noise. A DSL line driver 12is designated by a triangular “T” reference, while a DSL line receiver14 is designated by a triangular “R” reference. A disturbing circuit 16is shown as both a NEXT noise component source and a FEXT noisecomponent source. NEXT noise occurs when the line receiver 14 of thedisturbed circuit 20 is located at the same end of the cable bundle 10as the line driver 12. The disturbed circuit 20 experiences NEXT noisedue to electromagnetic radiation received on line (or loop) 26 fromline/loop 24 in the cable bundle 10. FEXT noise occurs when the linereceiver 14 of the disturbed circuit 18 is located at the other end ofcable bundle 10 from the line driver 12. The disturbed circuit 18experiences FEXT noise due to electromagnetic radiation received online/loop 22 from line/loop 24. The NEXT noise component is generally ofmuch greater magnitude and concern than the FEXT noise component.

In order to improve the performance of a DSL modem, one primaryobjective of a modem designer should be the minimization of thecrosstalk noise in the cable bundle 10. This is especially true withrespect to the NEXT noise component. For example, NEXT noise may beminimized in prior art G.Lite and G.DMT DSL system implementations byseparating the upstream and downstream bandwidths. This prior artsolution, however, is of limited utility as DSL modems andcommunications services become more complex, and a need exists for atechnique of more universal and future applicability for reducing theNEXT noise component and combating power dissipation concerns.

In accordance with the present invention, an optimized crosstalkperformance for a DSL system may be obtained by considering thefollowing factors:

Minimization of the NEXT noise. The existence of overlapping upstreamand/or downstream bandwidths for DSL communications by plural users on acommon cable bundle is a primary cause of NEXT noise. It is furtherrecognized, as discussed above, that not all of the availableupstream/downstream bandwidth is needed and thus a smaller, necessary orrequired bandwidth may be allocated. Some control may be exercised overthe placement of the required downstream bandwidth within the DSLspectrum. By selectively placing the required downstream bandwidth, theNEXT effect experienced by others on the same cable bundle as a resultof a common or overlapping bandwidth between loops may be minimized, andsignificant reductions in NEXT noise may be achieved.

Minimization of allocated bandwidth. DSL operation dictates theinsertion of idle ATM cells to fill all data frames when the bit rate ofthe data to be transmitted is smaller than the available throughput rateof the DSL link (both upstream and downstream) that is defined by theallocated number of subcarriers. For example, when the DSL user isbrowsing a website, the upstream data rate can be as low as few kilobitsper second, with the remainder of the available throughput rate is metby the transmission of idle ATM cells that add substantively nothing tothe data transmission but nonetheless contribute significantly tocrosstalk noise as well as power consumption. In some extremesituations, for example, when there is no data to be transmitted, idleATM cells are transmitted to fill the available throughput rate andaccordingly comprise the only source of crosstalk noise. Byintelligently selecting the minimum number of the subcarriers used forthe Digital Multi-tone (DMT) signals (i.e., minimizing the utilizedbandwidth) according to the bit rates of the data streams in theupstream and downstream directions, the size of the DSL link bandwidthused for communication is better tailored to the data being transmittedand crosstalk noise to other users, especially NEXT noise, can besignificantly reduced. As an added benefit, by controlling the usage ofthe upstream and downstream bandwidth in terms of the minimum number ofallocated and utilized DMT subcarriers, the power consumption of theline driver is substantially reduced.

Attention is now directed to FIG. 4 which is a flow diagram illustratinga process for NEXT noise minimization when establishing a new DSL loopcommunication. In step 90, the required bit rate for the datacommunication (upstream and/or downstream) over the new DSL loopcommunication is determined. More specifically, the data communicationis examined to identify and remove idle ATM cells. What is left oversubstantially represents the bit rate requirements for transmission ofthe data communication itself.

An idle ATM cell removal process performed in connection with step 90permits the identification ATM cell boundaries in the payload of thedata communication. The cells within the boundaries may then bediscarded. Reference is now made to FIG. 5 wherein there is shown astate machine diagram operating the idle ATM cell removal process. Thedetails of the state diagram are described below.

In the HUNT state, the ATM delineation process is performed by checkingbit-by-bit for the correct header error control (HEC) field in the cellheader. Once it is found, an assumption is made that one header has beenfound, and the method enters the PRESYNC state. It should be recognizedthat when byte boundaries are available, the cell delineation processmay be performed on a byte-by-byte basis instead.

In the PRESYNC state, the delineation process is performed by checkingcell-by-cell for the correct HEC field. The process repeats until thecorrect HEC field has been confirmed a certain number (designated DELTA)of times consecutively. As an example, ITU-T I.432 suggests that theDELTA number be 6. The process then moves to the SYNC state. If anincorrect HEC field is found, the process returns to the HUNT state.

In the SYNC state, idle cells will be discarded by checking the headerof each cell. The process for performing this discarding operation isshown in the flow diagram of FIG. 6. The cell delineation will beassumed to be lost if an incorrect HEC field is obtained a certainnumber (designated ALPHA) of times consecutively. As an example, ITU-TI.432 suggests that the ALPHA number be 7. If an incorrect HEC field isfound, the process returns to the HUNT state.

The idle cell discarding operation of FIG. 6 that is performed in theSYNC state first reads an ATM cell in (step 100). Next, in step 102, theread-in cell is error checked using the HEC field. If the number oferror bits exceeds one, as determined in step 104, the process performedby the idle cell removal machine will report the HEC error and return tostep 102. Otherwise, the process moves on to check in step 106 for anidle ATM cell by, for example, determining whether the virtual pathidentifier (VPI) virtual channel identifier (VCI) and payload type (PLT)information bits in the header are all zero, and also if the cell losspriority (CLP) is one. If not all of these conditions are met, nothingis to be done with the read-in cell (step 108) and the process returnsto step 102. The reason for this is that the process for idle cellremoval is designed to only remove the redundant idle cells in the ATMdata stream. Original data must remain unchanged. If the ATM cell underexamination is not an idle cell (i.e., the cell is a data cell), thecell is passed on to the next transmitter processing stage. If there isa match in step 106, the process moves to step 110 where a determinationis made as to whether no data exists to be transmitted on the link. Ifthe determination is no (i.e., that there is data to be transmitted),then the ATM cell can be discarded in step 112. Otherwise, the ATM cellis kept in step 114 as a minimum required ATM cell for the link (forexample, for DSL synchronization purposes). Following steps 112 or 114,the process returns to step 100 to read in a next ATM cell.

Reference is now once again made to FIG. 4. The step 90 determined bitrates for the upstream and downstream will generally be much smallerthan the corresponding maximum available throughput rates of the DSLloop. This allows for some flexibility to be exercised in selectivelyusing different parts of the available upstream and downstream bandwidthto minimize instances of overlapping bandwidth within the same cablebundle that contributes to NEXT noise and further reduce the powerconsumption of the DSL modem. The operation for selective bandwidthutilization is performed in step 92.

The concept of selective bandwidth utilization (step 92) is illustratedin an exemplary fashion in FIG. 7 for a CPE DSL receiver. Trapezoid 70represents the total available upstream bandwidth. Trapezoid 72represents the total available downstream bandwidth. Shaded trapezoid 74represents the required downstream bandwidth needed to supporttransmission of the step 90 determined downstream bit rate for a DSLcommunication. It is recognized that the required downstream bandwidth74 is smaller than the total available downstream bandwidth 72. Becauseof this, a selective position placement of the required downstreambandwidth 74 within the total available downstream bandwidth 72 may bemade in step 92. This selective placement is effectuated by sliding (asindicated by the arrows 76) the required downstream bandwidth 74 inposition along the frequency axis until a suitable location isidentified. The determination of what is suitable is made in accordancewith the present invention by evaluating crosstalk noise at eachpotential required downstream bandwidth 74 location within the totalavailable downstream bandwidth 72. The location chosen for thepositioning and placement of the required downstream bandwidth 74 withinthe total available downstream bandwidth 72 is that location wherecrosstalk noise due to overlapping bandwidth is minimized.

It is to be noted here that the meaning of “overlapping bandwidth” inthe context of the present invention is the bandwidth that isresponsible for the existence of crosstalk noise (primarily, NEXT noise)in a cable bundle. This is graphically illustrated in FIGS. 8 and 9 fortwo different modes of DSL operation. In FIG. 8, selective bandwidthutilization to minimize NEXT noise is illustrated in a non-overlappedDSL system implementation. A non-overlapped DSL system is one where theupstream and downstream bandwidths are separated from each other in thefrequency band. In FIG. 9, selective bandwidth utilization to minimizeNEXT noise is illustrated in an overlapped DSL system implementation. Anoverlapped DSL system is one where the upstream and downstreambandwidths are not separated from each other (i.e., they wholly orpartially overlap) in the frequency band.

Turning first to FIG. 8, it is noted that on a given cable bundle, Mnon-overlapped loop communications 80(1)-80(M) already exist. Thesecommunications 80 are established and are each using a designatedupstream bandwidth 82 and a designated downstream bandwidth 84 on theirrespective individual loops. Notably, these bandwidths may, and likelywill be, of different sizes and positions within the frequency band. Atthis point, a new loop communication 86 is to be initiated. This loopcommunication 86, as discussed above in connection with FIG. 7, has atotal available upstream bandwidth 70, a total available downstreambandwidth 72, and a required downstream bandwidth 74 that is needed tosupport transmission of the FIG. 4, step 90, determined downstream bitrate.

With respect to the DSL receiver for the new loop communication 86, thegroup of loop communications 80(1)-80(M) and 86 have a NEXT noiseoverlapping bandwidth 88 extending in the frequency band from f_(L) tof_(H) for downstream communications within the cable bundle. Thefrequency f_(L) at the low end of the NEXT noise overlapping bandwidth88 is the lowest frequency for any of the downstream bandwidths 72 or 84in same cable bundle. The frequency f_(H) at the high end of the NEXTnoise overlapping bandwidth 88 is the highest frequency for any of theupstream bandwidths 70 or 82 in same cable bundle. Noting again that thetotal available downstream bandwidth 72 is wider than the requireddownstream bandwidth 74, there exist several (if not many) possiblelocations where the required downstream bandwidth can be placed withinthe total available downstream bandwidth. It is further recognized thatthe NEXT noise contributed to the cable bundle by the addition of thenew loop communication 86 and its required downstream bandwidth 74varies as a function of position within the total available downstreambandwidth 72. Theoretically speaking, a best location for the requireddownstream bandwidth 74 would be completely outside the NEXT noiseoverlapping bandwidth 88 (for example, in the region designated atreference 116). In most situations, however, due to the relative sizesof the required downstream bandwidth 74 and the total availabledownstream bandwidth 72, as well as the sizes and positions of thebandwidths 82 and 84, this may not be achievable. However, by slidingthe position of the required downstream bandwidth 74 within the totalavailable downstream bandwidth 72 and through the NEXT noise overlappingbandwidth 88 as indicated by the arrows 76, and further noting the NEXTnoise contributed to the cable bundle at each possible location, anoptimal position having minimized NEXT noise effect may be selected forthe required downstream bandwidth 74.

Turning next to FIG. 9, it is noted that on a given cable bundle, Moverlapped loop communications 120(1)-120(M) already exist. Thesecommunications 120 are established and are each using a designatedupstream bandwidth 122 and a designated downstream bandwidth 124.Notably, these bandwidths may, and likely will be, of different sizes,and further overlap each other in whole or in part within the frequencyband. At this point, a new loop communication 126 is to be initiated.This loop communication 126, as discussed above in connection with FIG.7, has a total available upstream bandwidth 70, a total availabledownstream bandwidth 72, and a required downstream bandwidth 74 that isneeded to support transmission of the FIG. 4, step 90, determineddownstream bit rate. Note, in this scenario, that the total availableupstream bandwidth 70 and the total available downstream bandwidth 72may overlap each other in whole or in part within the frequency band.

With respect to the DSL receiver for the new loop communication 126, thegroup of loop communications 120(1)-120(M) and 126 have a NEXT noiseoverlapping bandwidth 128 extending in the frequency band from f_(L) tof_(H) for downstream communications within the cable bundle. Thefrequency f_(L) at the low end of the NEXT noise overlapping bandwidth128 is the lowest frequency for any of the downstream bandwidths 72 or124 in same cable bundle. The frequency f_(H) at the high end of theNEXT noise overlapping bandwidth 128 is the highest frequency for any ofthe upstream bandwidths 70 or 122 in same cable bundle. Noting againthat the total available downstream bandwidth 72 is wider than therequired downstream bandwidth 74, there exist several (if not many)possible locations where the required downstream bandwidth can be placedwithin the total available downstream bandwidth. It is furtherrecognized that the NEXT noise contributed to the cable bundle by theaddition of the new loop communication 126 and its required downstreambandwidth 74 varies as a function of position within the total availabledownstream bandwidth 72. Theoretically speaking, a best location for therequired downstream bandwidth 74 would be completely outside the NEXTnoise overlapping bandwidth 128 (for example, in the region designatedat reference 116). In most situations, however, due to the relativesizes of the required downstream bandwidth 74 and the total availabledownstream bandwidth 72, as well as the sizes and positions of thebandwidths 122 and 124, this may not be achievable. However, by slidingthe position of the required downstream bandwidth 74 within the totalavailable downstream bandwidth 72 and through the NEXT noise overlappingbandwidth 128 as indicated by the arrows 76, and further noting the NEXTnoise contributed to the cable bundle at each possible location, anoptimal position having minimized NEXT noise effect may be selected forthe required downstream bandwidth 74.

Although FIGS. 7-9 illustrate operation of the selective bandwidthutilization process with respect to a DSL receiver and the downstreambandwidth, it will be understood by those skilled in the art that asimilar operation may be implemented with respect to positioning arequired upstream bandwidth within a total available upstream bandwidthas well. It should also be understood and recognized that the process isequally applicable to the DSL modem at either end of the cable bundle.The specific reference and illustration in FIGS. 7-9 to downstreambandwidth is exemplary in nature only.

In order to make the position determinations discussed above inconnection with FIGS. 8 and 9, and execute the performance of the FIG.4, step 92, selective bandwidth utilization process, the NEXT noise ateach location, as the position of the required bandwidth (for example,reference 74) is slid within the total available bandwidth (for example,reference 72), must be computed. The computation of NEXT noise may beaccomplished using either of the following two different methods:

First, the Analytical Method. The NEXT noise from n identical disturbingsources can be modeled with empirical coupling transfer functions of thefollowing form (Equation 1):PSD_(NEXT)(ƒ_(k))=PSD_(disturber)(ƒ_(k))×X _(N) ×n ^(0.6)×ƒ^(3/2)

-   -   wherein: X_(N)=8.536×10⁻¹⁵;        -   n=number of disturbers;        -   f_(k) is the frequency in Hz at k-th subcarrier; and        -   PSD_(disturber) is the power spectrum of the interfering            system.            See, T1.417, Spectrum Management for Loop Transmission            Systems, American National Standard, Alliance for            Telecommunications Industry Solutions (ATIS), January 2001.            However, it is very common that different disturbers            co-exist in the same cable. To combine the crosstalk            contributions from different disturbers, the following            expression is used to calculate the NEXT noise due to the            combination of sources (Equation 2):

${{PSD}_{NEXT\_ TOTAL}\left( f_{k} \right)} = \left( {\sum\limits_{i}^{M}\left( {{PSD}_{i,{disturber}}\left( {f_{k},n_{i}} \right)} \right)^{\frac{1}{0.6}}} \right)^{0.6}$

-   -   wherein: M is the number of the types of the disturbers; and        -   n_(i) is the number of the disturbing sources for each type.            See, T1.417, Spectrum Management for Loop Transmission            Systems, American National Standard,. Alliance for            Telecommunications Industry Solutions (ATIS), January 2001.            For example, consider the case of two sources of NEXT at a            given receiver. In this case there are n₁ disturber systems            of spectrum S₁(f) and n₂ disturber systems of spectrum            S₂(f). The combined NEXT is accordingly expressed as            (Equation 3):

${{PSD}_{NEXT\_ TOTAL}\left( f_{k} \right)} = \left( {\left( {S_{1}\left( {f_{k},n_{1}} \right)} \right)^{\frac{1}{0.6}} + \left( {S_{2}\left( {f_{k},n_{2}} \right)} \right)^{\frac{1}{0.6}}} \right)^{0.6}$See, T1.417, Spectrum Management for Loop Transmission Systems, AmericanNational Standard, Alliance for Telecommunications Industry Solutions(ATIS), January 2001.

Second, the Estimation Method. To compute the NEXT noise, an estimatecan be made by evaluating the silent symbols during the initializationprocess. The corresponding equation for this action is as follows(Equation 4):

${{PSD}_{NEXT\_ TOTAL}\left( f_{k} \right)} = {\frac{1}{L\sqrt{2N}}{\sum\limits_{i = 0}^{L - 1}{\sum\limits_{n = 0}^{{2N} - 1}{{r_{i}(n)} \cdot {\exp\left( \frac{{j\pi}\;{kn}}{N} \right)}}}}}$

-   -   wherein: L is the total number of the silent DMT symbols for the        NEXT noise estimation;        -   i is the index of the subcarriers for NEXT estimation;        -   N is the maximum number of subcarriers the IDFT modulator            can support; and        -   the value r_(i)(n) is the n-th received sample for the i-th            DMT symbol.            It is to be noted here that this estimation result in fact            is the combination of NEXT, FEXT and additive white Gaussian            noise. However, as the NEXT noise is the major source of            interference, the above estimation can be approximately            regarded as the NEXT noise component.

Reference is now made to FIG. 10 wherein there is shown a flow diagramfor a process to minimize NEXT noise for the new initialized DSL loopcommunication in the same cable bundle in connection with making theposition determinations discussed above in connection with FIGS. 8 and9, and the execution of the FIG. 4, step 92, selective bandwidthutilization process. A loop 156 is executed to make calculations at aplurality of position locations. For each pass through the loop 156, atstep 150, the number of subcarriers needed to support the upstream datacommunication and downstream data communication is found. This step, ineffect, calculates the number of subcarriers for each position index ias the required bandwidth (for example, reference 74) is slid across thetotal available bandwidth (for example, reference 72). This numberrepresenting the number of needed subcarriers is likely to be differentat different positions (i.e., locations) of the required bandwidth dueto the fact that different numbers of bits can be supported in DSLsystem at different subcarriers. In step 152, the total NEXT noisecontributed by the required bandwidth at the current position locationis determined. This step, in effect, calculates the NEXT noisecontribution for each position index i as the required bandwidth (forexample, reference 74) is slid across the total available bandwidth (forexample, reference 72). This total NEXT noise calculation can bedetermined in accordance with the following (Equation 5):

$P_{NEXT} = {\sum\limits_{k = {k_{1}{(i)}}}^{k_{2}{(i)}}{{PSD}_{NEXT\_ TOTAL}\left( f_{k} \right)}}$

-   -   wherein: k₁ and k₂ are the beginning and ending points of the        required bandwidth in terms of the subcarrier index.        It is to be noted here that k₁ and k₂ will depend on the        position index i for the subcarriers. The number of subcarriers        between k₁(i) and k₂(i) is fully dependent on the        upstream/downstream bitmap for the DMT modulator. Finally, in        step 154, minimization of the NEXT noise is made by choosing the        position index i having the minimum value of P_(NEXT). The        corresponding k₁(i) and k₂(i) values represent the starting and        ending subcarriers for the required bandwidth (within the total        available bandwidth) at the determined position having minimum        NEXT noise.

It should be noted here that NEXT noise minimization process has anadded benefit in that the determined k₁(i) and k₂(i) values whichrepresent the starting and ending subcarriers of the required bandwidthat the NEXT noise minimized position within the total availablebandwidth further specify, for minimized NEXT noise, a minimum number ofsubcarriers that are necessary to carry the FIG. 4, step 90, determinedrequired bit rate for the data communication on the new DSL loop.Minimization of the transmission bandwidth with a smaller number of DMTsubcarriers leads to a reduction in the power consumption of the linedriver. The DMT signal samples in real form after the IDFT modulationcan be expressed as (Equation 6):

${s(n)} = {\sqrt{\frac{2}{N}}{\sum\limits_{k = 1}^{N - 1}{g_{k}\left\{ {{a_{k}{\cos\left( \frac{\pi\;{kn}}{N} \right)}} + {b_{k}{\sin\left( \frac{\pi\;{kn}}{N} \right)}}} \right\}}}}$

-   -   wherein: a_(k)-jb_(k) is the transmitted data for the k-th        sub-carrier;        -   N is the maximum number of the subcarriers the IDFT            modulator can support;        -   2N is the fast Fourier transform size of the DMT system; and        -   g_(k) is the transmission power control factor for the k-th            subcarrier.            The average power of the DMT signal can be easily determined            as follows (Equation 7):

$P_{s}^{2} = {\frac{1}{N}{\sum\limits_{k = 1}^{N - 1}{g_{k}^{2}\left( {a_{k}^{2} + b_{k}^{2}} \right)}}}$However, if not all of subcarriers are used in the transmitter, theaverage power of the DMT signal becomes (Equation 8):

$P_{s}^{2} = {\frac{1}{N}{\sum\limits_{k = k_{1}}^{k_{2}}{g_{k}^{2}\left( {a_{k}^{2} + b_{k}^{2}} \right)}}}$

As the bit rates may vary significantly for different applications andthe data rate across the network has bottlenecks, the operationdisclosed above for selective bandwidth utilization will have asubstantial effect on power consumption reduction. For example, if weassume the downstream bit rate is 500 Kb/s, which is typically notavailable as an Internet accessing speed for most residential users, thepower consumption can be reduced by minimizing the number of subcarriersby approximately 91.66% ((6000−500) Kb/6000 Kb). Here, we assume athroughput for the DSL downstream of 6 Mb/s. For a downstream connectionwith a lower available accessing speed, this figure can still be higher.

Reference is now once again made to FIG. 4. Having determined the sizeand location of the required bandwidth within the total availablebandwidth for the new DSL communication, the process generates of theNEXT minimized digital multi-tone (DMT) signal in step 94.

The maximum number of the available upstream (U) and downstream (D)subcarriers (S) that can be supported by a DSL modem is denoted asN_(SU) and N_(SD), respectively. It is noted that N_(SU) and N_(SD)might be different for various DSL standards. It is also noted that notall the available subcarriers are actually used in connection with theimplementation of the present invention. The number of the subcarriersactually used for the upstream and downstream are accordingly denoted asN_(upstream) and N_(downstream). The N_(upstream) and N_(downstream)subcarriers are determined in the manner set forth above (using theprocess of step 92 and the determination of the position index i havingthe NEXT noise minimum value of P_(NEXT) along with the correspondingk₁(i) and k₂(i) values representing the starting and ending subcarriersfor the required bandwidth). As also discussed above, the determinationof the actual number of subcarriers used is dependent on actual datarate to be transmitted by the DSL modem (upstream and downstream) asdetermined in step 90.

Reference is now made to FIGS. 1 and 2 which illustrate functional blockdiagrams of ATU transmitters in accordance with embodiments of thepresent invention. The ATU-R transmitter is shown in FIG. 1 and theATU-C transmitter is shown in FIG. 2. The ATU-R transmitter is similarto ATU-C transmitter but without need for and use of an Operation,Administration and Maintenance (OAM) path. The general configuration andoperation of such DSL transmitters is well known to those skilled in theart. More detailed discussion of the transmitters is made only to theextent necessary to understand operation of the present invention. Theoperations of steps 90 and 92 as set forth in FIG. 4 may be performed bythe Mux/Sync Control/Idle Cell Removal machine 196 within FIGS. 1 and 2.

Reference is now made in combination to FIGS. 1, 2 and 4. The generationof the NEXT minimized DSL signal in step 94 is realized through inverseDiscrete Fourier Transform (IDFT) modulation (reference 190). Themodulating transformation that defines the relationship between the realtime domain samples x_(n) (i.e., the DSL output signals) and the IDFTinput Z_(i)′ (to be discussed below) is given by (Equation 9):

$x_{n} = {\sum\limits_{i = 0}^{{2N} - 1}{{\exp\left( \frac{j\;\pi\;{ni}}{N} \right)}Z_{i}^{\prime}}}$

-   -   wherein: n=0, . . . , 2N−1;        -   N is a general symbol for the maximum number of the            subcarriers supported by the modem (either N_(SU) or            N_(SD)); and        -   i denotes the subcarrier whose real time domain samples            x_(n) are being calculated.            The result X_(n) is the signal to be transmitted on the new            DSL loop 192. When the steps 90 and 92 are performed by the            ATM idle cell removal machine within the MUX/Sync/Idle Cell            Remove block 196 in FIGS. 1 and 2 of the transmitter model,            the output x_(n) from the IDFT modulator 190 should generate            less NEXT noise to other users in the cable bundle and be            power reduced.

For ATU-R (residential or CPE location) and ATU-C (CO location)transmitters, Z_(i)′ is generated using different methods as discussedin more detail below.

For an ATU-R transmitter, assume that N_(upstream) subcarriers (k₁(i),k₁(i)+1, . . . , k₂(i), where k₂(i)=k₁(i)+N_(upstream)−1) are allocatedfor the transmission of an upstream signal for a given bit rate. Therelationship between k₁(i), k₂(i), and N_(SU) are: k₁(i)≧1 andN_(SU)>k₂(i)>k₁(i). It should be noted, for the convenience of thisdiscussion, that it is assumed that the N_(upstream) subcarriers arecontinuous subcarriers in frequency domain, but this is not a necessity.The complex values from the constellation encoder and gain scaling(reference 194) for the i-th subcarrier is Z_(i) (i.e., the input datato be transmitted, already packed into symbols). In order to generatethe real output x_(n) from the IDFT modulation as set forth above, Z_(i)is first mapped to Z_(i)′ using (Equation 10):

$Z_{i}^{\prime} = \left\{ \begin{matrix}0 & {0 \leq i < {k_{1}(i)}} \\Z_{i} & {{k_{1}(i)} \leq i \leq {k_{2}(i)}} \\0 & {{{k_{2}(i)} + 1} \leq i \leq N_{SU}}\end{matrix} \right.$The vector Z_(i)′ shall be augmented such that Z_(i)′ has the Hermitiansymmetry as follows (Equation 11):

Z_(i)^(′) = conj(Z_(2N_(SU) − i)^(′))for  i = N_(SU) + 1  to  2N_(SU) − 1.Equation (9) is then used by reference 190 to generate the real outputx_(n) from vector Z_(i)′.

For an ATU-C transmitter, assume N_(downstrean) subcarriers (k₁(i),k₁(i)+1, . . . , k₂(i), wherein k₂(i)=k₁(i)+N_(downstream)−1) areallocated for the transmission of the downstream signal for a givenrate. The relationship between k₁(i), k₂(i), N_(SU) and N_(SD) are:k₁(i)≧N_(SU)+1; and N_(SD)>k₂(i)>k₁(i). As we mentioned earlier, theN_(downstream) subcarriers allocated to the downstream need notnecessarily be continuous. A non-overlapped spectrum is assumed in theDSL system operation for this discussion. In the overlapped mode, thegeneration of the DSL downstream signal is similar to the upstream. Inorder to generate real output x_(n) from IDFT modulation (as discussedabove), Z_(i) is first mapped to Z_(i)′ using (Equation 12):

$Z_{i}^{\prime} = \left\{ \begin{matrix}0 & {0 \leq i < {k_{1}(i)}} \\Z_{i} & {{k_{1}(i)} \leq i \leq {k_{2}(i)}} \\0 & {{{k_{2}(i)} + 1} \leq i \leq N_{SD}}\end{matrix} \right.$The vector Z_(i)′ shall be augmented such that Z_(i)′ has Hermitiansymmetry as follows (Equation 13):

Z_(i)^(′) = conj(Z_(2N_(SD) − i)^(′)) for i = N_(SD) + 1 to 2N_(SD) − 1.

Equation (9) is then used by reference 190 to generate the real outputx_(n) from vector Z_(i)′.

Although preferred embodiments of the method and apparatus of thepresent invention have been illustrated in the accompanying Drawings anddescribed in the foregoing Detailed Description, it will be understoodthat the invention is not limited to the embodiments disclosed, but iscapable of numerous rearrangements, modifications and substitutionswithout departing from the spirit of the invention as set forth anddefined by the following claims.

1. A method for optimizing digital subscriber line (DSL) communicationsperformance over a cable bundle having at least a first loop and asecond loop and including at least an active DSL loop communication onthe first loop, comprising the steps of: performing a DSL loopinitialization process with respect to a new DSL loop communication onthe second loop of the cable bundle, the initialization processincluding: identifying a group of plural subcarriers numbering less thana total number of available subcarriers on the second loop which areneeded for carrying a required bit rate of the new DSL loopcommunication; sliding the identified group of plural subcarriers acrossa frequency band that includes the total available subcarriers;calculating, at each of a plurality of location positions within thefrequency band for the slid identified group of plural subcarriers, acrosstalk noise contribution of the new DSL loop communication to thecable bundle, wherein the crosstalk noise contribution varies as afunction of location position; and choosing one of the locationpositions for the slid identified group of plural subcarriers, whereinthe chosen location position is the location position where thecalculated crosstalk noise contribution to the cable bundle by the newDSL loop communication is lowest; and generating a DMT signal using theidentified group of plural subcarriers positioned at the chosen locationposition to carry the new DSL loop communication.
 2. The method as inclaim 1 wherein a number of subcarriers needed for the group of pluralsubcarriers varies with the plurality of location positions for thegroup of plural subcarriers within the total available subcarriers. 3.The method as in claim 1, wherein the process for initializationincludes silent DMT symbols, and wherein calculating the crosstalk noisecontribution comprises evaluating the silent DMT symbols in the contextof making a NEXT noise estimation.
 4. The method as in claim 1 whereinthe group of plural subcarriers is a group of upstream subcarriers andthe total available subcarriers is a total available upstream subcarriers.
 5. The method as in claim 1 wherein the group of pluralsubcarriers is a group of downstream subcarriers and the total availablesubcarriers is a total available downstream subcarriers.
 6. The methodas in claim 1 wherein the step of identifying further comprises the stepof removing unnecessary idle ATM cells, and the required bit rate forthe DSL loop communication is a bit rate needed for data communicationover the DSL loop without inclusion of unnecessary idle ATM cells. 7.The method as in claim 1 wherein the crosstalk noise contribution isnear-end crosstalk (NEXT) noise contribution.
 8. The method as in claim1 wherein the group of subcarriers comprises a group including at leasttwo adjacent subcarriers.
 9. The method as in claim 1 wherein the groupof subcarriers comprises a group including at least two non-adjacentsubcarriers.
 10. A digital subscriber line (DSL) transmitter adapted forconnection to the second loop in the cable bundle and configured toimplement the process of claim
 1. 11. A method for optimizing digitalsubscriber line (DSL) communications performance over a cable bundlehaving at least a first loop and a second loop and including at least anactive DSL loop communication on the first loop, comprising the stepsof: performing a DSL loop initialization process with respect to a newDSL loop communication on the second loop of the cable bundle, theinitialization process including: a) identifying a first group of pluralsubcarriers extending between first and second subcarrier locationpositions in a frequency band and numbering less than a total availablesubcarriers on the second loop of the cable bundle which have a capacityfor handling a bit rate of the new DSL loop communication; b)calculating for the first group of plural subcarriers a crosstalk noisecontribution of the new DSL loop communication to the cable bundle; c)identifying a second group of plural subcarriers extending between thirdand fourth subcarrier location positions in the frequency band andnumbering less than the total available subcarriers on the second loopof the cable bundle which have a capacity for handling the bit rate ofthe same new DSL loop communication; d) calculating for the second groupof plural subcarriers a crosstalk noise contribution of the same new DSLloop communication to the cable bundle; and e) identifying which of thefirst and second group of plural subcarriers the group whose calculatedcrosstalk noise contribution to the cable bundle is least; andgenerating a DMT signal using the identified group of plural subcarriersto carry the new DSL loop communication.
 12. The method as in claim 11wherein a number of subcarriers needed for the identified first andsecond groups of plural subcarriers is different.
 13. The method as inclaim 11 wherein the first and second groups of plural subcarriers areupstream subcarriers and the total available subcarriers is a totalavailable upstream subcarriers.
 14. The method as in claim 11 whereinthe first and second groups of plural subcarriers are downstreamsubcarriers and the total available subcarriers is a total availabledownstream subcarriers.
 15. The method as in claim 11 wherein the stepsof identifying comprise removing unnecessary idle ATM cells, and therequired bit rate for the DSL loop communication is a bit rate neededfor data communication over the DSL loop without inclusion ofunnecessary idle ATM cells.
 16. The method as in claim 11 wherein thecrosstalk noise contribution is near-end crosstalk (NEXT) noisecontribution.
 17. The method as in claim 11 wherein the first and secondgroups of subcarriers each comprise a group including at least twoadjacent subcarriers.
 18. The method as in claim 11 wherein the firstand second groups of subcarriers each comprise a group including atleast two non-adjacent subcarriers.
 19. The method of claim 11 whereinthe subcarrier location positions are defined by an index, the methodfurther comprising increasing the index to change between subcarrierlocation positions.
 20. The method of claim 19 wherein increasing theindex slides the group of plural subcarriers over the frequency band.21. The method of claim 11, wherein the process for initializationincludes silent DMT symbols, and wherein calculating the crosstalk noisecontribution comprises evaluating the silent DMT symbols in the contextof making a NEXT noise estimation.
 22. A digital subscriber line (DSL)transmitter adapted for connection to the second loop in the cablebundle and configured to implement the process of claim 11.